The present invention relates to a process for preparing vinyl alcohol copolymers which can give moldings having improved qualities, and more particularly to a process for preparing an ethylene-vinyl alcohol copolymer or an .alpha.-olefin-vinyl alcohol copolymer.
Various uses are expected from vinyl alcohol copolymers because of having various properties which are not found in a vinyl alcohol homopolymer due to kinds and amounts of monomers copolymerized with vinyl alcohol. Particularly, an ethylene-vinyl alcohol copolymer and an .alpha.-olefin-vinyl alcohol copolymer have excellent oxygen impermeability, oil resistance, antistatic property, mechanical strength, and the like, and are useful as wrapping or packaging materials in any shape such as sheet, film or container.
Recently, however, the requirement of high qualities is increasing with technical revolution and, for instance, presence of slight fish eyes or surface roughness on moldings of ethylene-vinyl alcohol copolymer becomes a problem and also a higher transparency is required.
That is, as the most general process for preparing ethylene-vinyl alcohol copolymer, there is a process in which ethylene and vinyl acetate are copolymerized by solution polymerization, then, removing unpolymerized vinyl acetate from the polymerization mixture, and hydrolysis reaction of the ethylene-vinyl acetate copolymer is carried out in the presence of an alkali catalyst. According to the above-mentioned process, when an amount of alkali catalyst is increased, reaction temperature is raised or reaction time is prolonged in order to obtain high hydrolysed copolymer, the obtained hydrolysed copolymer has defects that coloration, fish eyes, surface roughness remarkably occur on the melt moldings of the hydrolysed copolymer. Accordingly, it is attempted that the hydrolysed copolymer is sufficiently washed in comparison with a conventional technique, but the sufficient washing does not yet provide a satisfactory result in improving the above-mentioned defects.
The present inventor thought that the abovementioned defects would result from a polymerization inhibitor added to the solution of ethylene-vinyl acetate copolymer after copolymerization reaction and made an experiment on various polymerization inhibitors such as m-dinitrobenzene, hydroquinone, hydroquinone monomethyl ether, hydroquinone dimethyl ether, t-butyl catechol, nonyl phenol, o-benzoquinone, p-benzoquinone, thiodiphenylamine, sulfur, styrene, butadiene, naphthalene and anthracene. However, the present inventor could not find a polymerization inhibitor having effect of preventing the formation of fish eyes, coloration, and fumes or bad odors through molding in addition to the effect of inhibiting the polymerization. For instance, in case of employing m-dinitrobenzene as a polymerization inhibitor, when an amount of an alkali catalyst is increased in order to raise a degree of hydrolysis, the coloration becomes remarkable. On the other hand, when an amount of an alkali catalyst is decreased, even the coloration is decreased but the effect of inhibiting the polymerization is lacked and fish eyes in films of hydrolyzed copolymer increase.
An object of the present invention is to provide a process for preparing a vinyl alcohol copolymer which can give easily a molding having excellent qualities.
A further object of the present invention is to provide a process for preparing an ethylene-vinyl alcohol copolymer and an .alpha.-olefin-vinyl alcohol copolymer which can give moldings having excellent qualities.
These and other objects of the present invention will become apparent from the description hereinafter.